Manual de Plantas de Costa Rica 95 
For over 34 years Tosi’s ecological map (1969), based on the life-zone concept of 
Holdridge (1967), has been widely accepted and used as a model for the classification 
of vegetation types of the country. Costa Rica harbors 12 of the 37 life-zone classes in 
the Holdridge system; their descriptions and distributions can be found in Holdridge et 
al. (1971), Hartshorn (1983), and Hartshorn et al. (1982). Since the Manual uses the 
Holdridge system to characterize the habitat (forest type) of each species, we here at- 
tempt to provide a characterization of the patterns of vegetation related to these forest 
types. However, in the Manual treatments, and to a certain extent here, we use a sim- 
plified system of only five zones: dry forest (bosque seco), moist forest (bosque 
himedo), wet forest (bosque muy hiimedo), rain forest (bosque pluvial), and paramo 
(paramo). Occasional reference is also made to the Mapa de unidades bioticas de Costa 
Rica (Herrera & Gomez P., 1993), a new and different perspective for classification. 
As specified by Holdridge (1967), his system depends on three simple climatic 
measurements: average annual biotemperature, average annual precipitation, and aver- 
age potential evapotranspiration. Applied on a triangular diagram that consists of three 
components— latitudinal regions, altitudinal levels, and precipitation categories — 
these three climatic data points define 30 hexagonal life zones. The climatic data are 
based on a logarithmic progression in an isogonic array (Hartshorn, 1983). Given that 
a particular life zone at two distant points on the planet will certainly have different spe- 
cies, the Holdridge system avoids giving importance to specific floristic components. 
Nevertheless, ample field experience demonstrates that plant species are good indica- 
tors of the climatic and topographic characteristics of a particular site and can therefore 
be useful as such, over the range of the species. In this regard, certain species (e.g., 
those within the understory) are less affected by the external climate and for that rea- 
son less valuable as indicator species. 
In the field, clear delimitation of the Holdridge system categories is not always pos- 
sible. For example, investigations along an altitudinal transect (100—2600 m) on the 
Caribbean slope showed quantitative differences among the various life zones (Hart- 
shorn & Peralta, 1988). These authors found a gradual transition between premontane 
rain forest and montane rain forest within an altitudinal band of 300 m, whereas tran- 
sitions between other life zones were less obvious, and their limits did not coincide with 
those originally proposed by Tosi (1969). They also demonstrated that many tree species 
occur in more than one kind of forest and more than one life zone. Later, Lieberman et 
al. (1996) reaffirmed some of those findings, indicating that species composition varies 
continuously with elevation and lacks evidence of discrete floristic zones. An overall 
analysis of the Ecuadorian flora came to much the same conclusion, and demonstrated 
that any particular elevational zone shares many species with adjacent zones, so that dis- 
crete elevational zones marked by differences in species composition are not apparent 
(Jorgensen & Leén- Yanez, 1999). In addition, Boyle (1996) has shown that these non- 
discrete gradients in species composition change over latitude as well as altitude. 
